Abstract:

The Badain Jaran Desert, Inner Mongolia, China is an important window to study the aridity and other paleoenvironmental problems in Asia. Taking the Barunbori Tolgoi section in the southeastern margin of the desert as the research object, mineralogical characteristics of sediments, such as structural and compositional maturity, were thoroughly analyzed. Through detailed sample mineralogy under a microscope, semi-quantitative X-ray diffraction (XRD) test, and grain size analysis, the sedimentary environment in this area during the late Pleistocene was reconstructed. Microscopic characteristics of sediments indicate that the mineral debris deposited before 66.8 ka had good sorting, high roundness, and high stable mineral contents, indicating high structural and compositional maturity. After 66.8 ka, the mineral debris exhibited poor sorting, low roundness, and low stable mineral contents, whereas the number of clay minerals increased, indicating lower structural and compositional maturity. The XRD results show that the sediments before 66.8 ka were dominated by felsic minerals with a percentage of 95%-100%. After 66.8 ka, the number of clay minerals gradually increased, indicating that the environment was relatively warm and humid. The appearance of carbonate and evaporite minerals in the later period indicated that the lake water developed toward the direction of salinity. The grain size analysis results show that the sediments before 66.8 ka are dominated by medium sand, with an average content of 65.33%. After 66.8 ka, the sand becomes fine, with the content ranging from 56.32% to 61.51%. The mineral-sorting coefficient corresponding to the standard deviation of grain size data shows that the sorting coefficient of the sediments before 66.8 ka is 0.40-0.67, indicating good sorting. After 66.8 ka, the sediment-sorting coefficients ranged from 0.55 to 1.03, indicating poor sorting. The results show that there was a sudden sedimentary environment change from aeolian sediments to lacustrine sediments around 66.8 ka in the southeastern margin of the Badain Jaran Desert, indicating a significant increase in the lake area and water level. According to the macroscopic geomorphological characteristics and measured data in the field, the elevation of the two salt lakes on the west side of the Barunbori Tolgoi section is approximately 1275 m, whereas the elevation of the platform on the top of the section is 1294 m, which means that the water level rose at least 19 m during this period. The lacustrine platform was gradually deposited on the underlying dunes during the long lake period. The sedimentary environment change may represent a typical East Asian summer monsoon intensification event, which is speculated to be related to the warming of the low-latitude western Pacific Ocean.

Key words: late Pleistocene, mineral characteristics, depositional environment, Badain Jaran Desert